Holograms have come into wide usage as decorative indicia due to their unique visual appearance. In addition, the difficulty in making and reproducing holograms has made them a common authentication feature on items like credit cards, driver's licenses and access cards. Holograms have also been used as security features on high end products, making it harder to counterfeit these products.
The most common method of creating a hologram is to create a grating pattern in a surface so that particular structures become visible upon diffraction of light in the grating. U.S. Pat. No. 3,578,845 to Brooks et al. describes how diffraction gratings are typically generated. Currently, the most common way to generate holograms based on diffraction patterns is to emboss the diffraction patterns into a thermo-formable substrate such as an embossable polymer film. In order to enhance the reflectance in the grating, a high reflective material such as aluminum is typically deposited onto the grating.
To add to the complexity of holographic identifiers, it is desirable to maximize the size of the hologram. The use of an opaque metal layer as a reflectance enhancement layer, however, can limit the size of a hologram as an identifier. If the hologram starts to overlie information located behind the hologram, the opaque metal layer can block out the information below the hologram. U.S. Pat. Nos. 5,142,383 and 5,145,212 to Malik offer one solution to this problem. Malik discloses the deposition of a discontinuous reflective metal, where small coated and uncoated areas exist next to each. The coated areas are large enough to create enough reflectance for the grating, but are small enough to be seen as a uniform coating by the unaided human eye. In addition, the non-coated areas are also large enough to allow the information behind the holographic image to be readable, but small enough and close enough to appear uniform to the unaided human eye. The combination of both effects creates a semi-transparent layer that allows the hologram to be visible while at the same time allowing the information behind the hologram to be readable.
U.S. Pat. No. 4,856,857 to Takeuchi et al. discloses different methods to achieve semi-transparency. One method is to apply a layer of metal like aluminum with a thickness of less than 200 Angstroms. According to Takeuchi et al., at these thickness the reflective metal layer still has transparency. Another method of achieving semi-transparency of the reflective layer is to deposit a transparent material layer with a refractive index different than the hologram bearing substrate. The required difference in the refractive index as claimed by Takeuchi et al. is larger than 0.2.
Typical materials for transparent reflection enhancing layers are certain oxides such as WO3, SiO, TiO2, Al2O3 or Fe2O3 and ceramic materials such as ZnS. Currently, the use of ZnS as a transparent reflection enhancement layer in transparent holograms is the most widely used technology. Mixtures of materials disclosed by Takeuchi et al. are used as well. U.S. Pat. No. 5,513,019 to Cueli discloses mixtures of zinc sulfide (ZnS) and tungsten-oxide (WO3) for holograms.
U.S. Pat. Nos. 5,757,521 and 5,789,910 to Walters et al. discuss the patterned deposition of metal to achieve local transparency through holograms. The pattern also incorporates resonant structures that can be used to identify the structure with appropriate instruments.
Common to the procedures for applying a thin reflection enhancing material layer behind the embossed diffractive pattern is the use of a vacuum vapor deposition technique. In the vapor deposition techniques, the coating material is evaporated at low pressure, typically in the 1×10−5 Torr region, at elevated temperature. By passing the substrate through the vapor, the evaporated material condenses on the embossed substrate surface creating a thin layer of reflection enhancing material. The thickness of the deposited layer depends on several factors, for example, evaporation rate, vapor pressure and dwell time of the substrate in the vapor cloud.
Diffractive grating holograms achieve their high level of security partly due to the difficulty of producing original or distinct holographic patterns. However, the difficulty of producing original holograms can be a disadvantage if certain levels of variation in the holographic pattern are desired to generate discernable features in a small number of holograms. Also, although transparent holograms provide a high level of security by allowing larger and more complex holographic structures on identification articles, the most commonly used high refractive index material, zinc sulfide, is water-soluble. This can create a problem in many applications, for example, a driver's license, may be exposed to water (washing machines) by accident. This exposure can lead to dissolution of the ZnS and the loss of the holographic feature.